Muscular dystrophies, particularly those associated with disruption of the function of the dystrophin glycoprotein complex, are characterized by muscles that are sensitive to mechanical damage. A key feature in the mechanism of muscle damage is the loss of muscle fiber plasma membrane integrity which ultimately results in severe muscle fiber degeneration, loss of muscle fibers, and consequently loss of muscle mass and progressive weakness. Post-mitotic muscle fibers, like many cells, have a remarkable capacity to repair membrane lesions, but little is known about the mechanisms of membrane repair in muscle fibers. Mutations in the protein dysferlin, are associated with LGMD 2B and Myoshi Myopathy in humans. Due to the similarity of dysferlin to other ferlins and synaptotagmin, dysferlin is believed to be involved in membrane trafficking. Loss of dysferlin in mice appears to disrupt the normal membrane repair pathway. One of the major limitations in studying membrane repair is that most of the approaches to study membrane repair are indirect, only showing how well membranes can effectively exclude membrane impermeant probes but doesn't necessarily differentiate differences in the magnitude of membrane wounding versus differences in efficiency of repair. The overall goal of this proposal is to develop a novel set of live cell molecular probes to specifically label the membrane repair pathway in live muscle fibers and muscles, and study the mechanisms of membrane trafficking directly in muscle fibers in response to experimental and physiological injury. Our preliminary data using one of these novel reporters challenges the current model that pre-existing vesicle or organelle compartments containing dysferlin are responsible for repairing plasma membrane lesions. Therefore, the proposed work will focus on two major aims: 1) Develop a set of live cell molecular probes in living mice to study the membrane repair pathway under experimental and physiological muscle injury. 2) Dissect how the submembrane actin cytoskeleton contributes to membrane repair by recruiting dysferlin to the site of membrane injury. The long term goal is to identify the mechanisms of membrane repair that hopefully can be exploited or enhanced in order to repair muscle damage in several forms of inherited muscular dystrophy.